Fundamentally, objective measuring parameters for the determination of the organic and metabolic functions exist in medicine and in particular in emergency and intensive care and surgery. These parameters are for instance electro-chemical values. By way of example attention is drawn to the very important metabolism taking place under the participation of oxygen in tissues and organs for the supply of body cells with oxygen. Within the scope of the oxygen supply of the organism and the effects depending thereon, a lack of oxygen may for instance occur in a tissue, which will lead to a modification of the electrolytic activities on the cell membrane (ischemia). Such a lack of oxygen is accompanied by an increase of the potassium activity outside the cell, while simultaneously the sodium activity decreases and the tissue pH value drops. Thus, potassium and sodium activities as well as the tissue pH value are possible electrochemical measured values for the determination of the supply of a tissue with oxygen. Any modifications beyond this can equally be detected by way of electro-myelography (EMG) with the aid of enzyme electrodes for the detection of glucose, lactate or similar metabolic products by ampereometric sensors.
Pathologically conditioned edema in the tissue are a further example giving rise to disturbances of the microcirculation, i.e. of the capillary blood circulation, to a pathological modification in the capillary wall and the inter-cell chamber (interstitium) and thus to similar metabolic modifications as described above. Here, too, the above-mentioned measuring methods can furnish good clues for diagnosis and therapy.
For the determination of the above electro-chemical measured values mentioned by way of example, suitable measuring methods and apparatuses working on a potentiometric or ampereometric base have fundamentally been known for a long time. However, these measuring methods and apparatuses have essentially been applied only within the scope of laboratory tests, for which samples of the tissue of interest have to be available.
By comparison, with a view to emergency and intensive care and surgery it is desirable for such measuring methods to be applied "in vivo"--for instance directly during an operation--with the aid of suitable measuring apparatuses, so that the surgeon gets important prompt information on the efficiency of his steps and on the condition of the organ operated within the scope of "on-line" diagnostics.
An apparatus of the generic type developed to this end is already known from German patent 37 25 597. With this apparatus for measuring the ion activity, a sensor head with miniaturized measuring electrodes adapted to the type of ions to be determined can be applied to the surface of a tissue, for instance an organ surface, and the activity of the ions of interest can be measured with the aid of ion-selective membranes in the vicinity of the measuring electrodes.
Upon contact of the ion-selective membrane with the medium containing the corresponding ions, an electromotive force or an electro-chemical potential EMF can be measured by way of lines from the membrane, which is also given by the NERNST equation qualitatively reproduced as follows, EQU EMF=E.sub.0 -C T lg a.sub.ion
with E.sub.0 : reference potentional, C: scale factor, T: temperature and a.sub.ion : ion activity.
On the basis of this relation the ion activity can be detected based on a potentiometric measurement.
Applied in practise, the measuring apparatus basically known from the above-mentioned publication involves problems in particular with regard to calibration and any subsequent measurements. As it is, prior to being used for measuring, each measuring electrode of the sensor must be calibrated in a time-consuming way, i.e. a calibration curve must be prepared of the electromotive force as a function of the ion activity at a certain temperature or a number of certain temperatures. Proceeding from this calibration curve an electromotive force measured at a corresponding temperature can be converted into a corresponding ion activity. The necessary calibration is a complicated preparatory step for measuring, it is time-consuming and thus conflicting with real "on-line" diagnostics. Furthermore, the known sensor has implied regular subsequent calibrations to take place in between the measurements for correction of the measured values. Moreover, measurement in different temperature ranges is not possible with the known measuring apparatus without prior calibration at a corresponding measuring temperature, which complicates its use in organ transplantation surgery, where stored donated organs are kept strictly cooled.